1. Field of the Invention
The present invention relates to a line quality monitoring apparatus and method for monitoring the quality of a transmission signal in an optical communication system.
2. Description of the Related Art
An optical communication system uses the following method of monitoring the quality of a transmission signal. That is, the transmitting side sends a data frame appended with an error detection bit, and the receiving side checks the error detection bit of the received data frame. This method is very effective for quality monitor of a transmission signal with a predetermined transmission rate and transmission format. However, it is not easy for a wavelength multiplex optical transmission apparatus capable of multiplexing and transmitting signals to append an error detection bit on the transmitting side irrespective of the transmission rate and transmission format.
By contrast, as a method of monitoring the quality of a transmission signal on only the receiving side without using any error detection bit, a method shown in FIG. 1 is known. Referring to FIG. 1, a received optical signal is converted into a voltage signal via a light-receiving element 101 and current/voltage converter 102. The voltage signal is input to identifiers (A) 103 and (B) 104 in which different identification levels are set. These identifiers compare their identification levels with the level of the received signal to make identification in a phase of a clock extracted by a timing extraction unit 105. The outputs from the identifiers are input to an EX-OR gate 106, and undergo bit comparison. The output from the EX-OR gate 106 and the clock extracted by the timing extraction unit 105 are input to an error rate calculation unit 107 to calculate an error rate.
FIG. 2 shows the relationship between the input signal level and the identification level in the identifier. Both H- and L-levels of the input signal in an identification phase are respectively distributed about average values μH and μL at a given probability density. If Vth1 and Vth2 respectively represent the identification levels of the identifiers (A) 103 and (B) 104, when the level of an input signal in the identification phase falls within the range between Vth1 and Vth2, the output result of the identifier (A) 103 is H level, and that of the identifier (B) 104 is L level. Hence, the two identifiers output different identification results. FIG. 2 indicates an error detection range in the input signal by hatching.
FIG. 3 shows the bit comparison operation. If the identification results of the two identifiers match, the output from the EX-OR gate 106 changes to L level. However, if the identification results are different, the output from the EX-OR gate 106 changes to H level. Hence, when the bit comparison result of the EX-OR gate 106 is H level, the error rate calculation unit 107 counts that bit as an error bit. The error rate calculation unit 107 counts clocks extracted by the timing extraction unit 105, and divides the count result of the error bits by the clock count, thus calculating an error rate.
As techniques associated with a bit comparison scheme, various techniques are known in addition to the above example shown in FIG. 1. For example, Jpn. Pat. Appln. KOKAI Publication No. 2-142247 discloses a fundamental technique, which compares an input signal with a predetermined identification level to identify a code. On the other hand, Jpn. Pat. Appln. KOKAI Publication No. 3-140039 has a function of detecting noise components contained in an input signal by parallelly receiving the input signal by two identifiers (comparators). Also, Jpn. Pat. Appln. KOKAI Publication No. 2000-4260 discloses a technique for checking if a bit of an input signal is H or L level using two bit checking circuits by setting a voltage amplifier threshold value and phase threshold value.
For example, with the arrangement shown in FIG. 1, when the level difference between the two identification levels Vth1 and Vth2 is set to be sufficiently smaller than the input signal amplitude, an actual error rate can be monitored to some extent. However, a setting condition of the identification level difference, which is used to eliminate detection errors of an error rate detected by the bit comparison scheme is not defined.
FIG. 4 shows the calculation result of the relationship between an error rate BER upon identifying at an optimal identification level, and a detection error (DET−BER)/BER of the error rate BER detected by the bit comparison scheme. In this calculation, the mark ratio is set to be ½, and both H and L levels are normal distributions having an identical variance. Also, an optimal identification level Vopt (see FIG. 2) is an intermediate value of average values μH and μL of H and L levels. The identification level of the bit comparison scheme assumes a case wherein Vth1 is set as the optimal identification level Vopt, and a level difference ΔV between Vth1 and Vth2 is set in proportion to an input signal amplitude Vpp.
As can be seen from FIG. 4, if ΔV/Vpp is set to be 1%, the detection error is small for a signal with high line quality around an error rate 10−10, but the detection error becomes larger with worsening line quality. If ΔV/Vpp is set to be 2%, the detection error is small around an error rate 10−6, but the detection error becomes larger with changing line quality.
In this way, when the identification level difference ΔV is set in proportional to the input signal amplitude Vpp, line quality cannot be precisely monitored within a broad range (e.g., the error rate range from 10−12 to 10−3).